Crowding clearly reduces the fissioning rate of the planaria, Dugesia dorotocephala. This reduction seems to be the result of some water soluble substance and is not the result of slime, oxygen depletion, or carbon dioxide increase. At densities below 2.0 planaria per 100 ml. of water, reproduction is not affected by crowding. If planaria do not reproduce at their maximum rate all the time, but only reproduce to replace losses, then the effect of intraspecific competition and natural selection is reduced. Supposedly, without natural selection there would have been no evolution. If planaria living today in a protected environment can turn their reproduction off when a certain maximum but healthy density is reached; then it is quite conceivable that in the perfect creation before the Fall these animals could regulate their own numbers without the necessity of outside predation, starvation or disease.